The electrostatic painting or coating is a widely adopted painting technique in which an electrostatic-painting spray gun is used to coat an object with a paint with a high efficiency by charging fine particles of a paint sprayed are charged with a high-voltage static while forming an electric field on the object. The paints used in such electrostatic painting are generally classified, from the viewpoints of action and effect, into a solvent type high in electrical resistance, and a highly conductive type low in electrical resistance, such as a water-based paint or metallic paint. An appropriate one the painting methods and painting apparatuses should be selected for a selected one of such paints.
Because of the recent movement for being friendly to the earth, it has been demanded to limit the use of the solvent type paints containing a volatile organic compound, and use the water-based paints instead. In the case of the conventional electrostatic-painting spray gun, however, when sprayed fine particles of a paint are directly charged with a high-voltage static to improve the efficiency of electrostatic paint adhesion, the applied voltage goes to a grounded paint supply source via the paint particles, resulting in that no electrostatic effect can be assured and the high voltage arriving at the paint supply source will possibly cause a danger. On this account, there is used an approach to make an electrostatic spraying with the paint supply source being insulated from the ground potential while maintaining the high voltage. However, a vast amount of charge on the paint supply source will inevitably lead to an increased danger. To make a continuous work of painting in order to attain a higher industrial efficiency of painting, it will be necessary to use a paint feeder as disclosed in the Japanese Patent Application Laid Open No. 198228 of 1994. It is required to introduce a larger-scale painting apparatus not readily usable and of which the maintenance is not easy. These problems cause the electrostatic spraying of water-based and metallic paints not to have been more prevailing and the environmental pollution not to be limited or stopped early and effectively.
In the field of electrostatic painting. there are well known the direct charging technique in which fine particles of a paint are directly charged and also the external charging technique in which paint particles are not directly charged but they are charged when passing through an air space ionized by discharging from an external electrode disposed outside a sprayed area, carried on electric lines of force (electric field) formed toward a to-be-painted object, and adhere or stick to the latter. For example, electrostatic-painting painting apparatuses formed each integrally with a spray gun and cooperating with the latter are known from the disclosure in the Japanese Patent No. 2770079 and Japanese Patent Application Laid Open No. 213958 of 1995. The electrostatic-painting painting apparatus of the external charging type is such that a high voltage is applied to an external electrode disposed in a position off an area sprayed with fine particles of a paint from the spray gun to cause discharging at the ground potential side of a to-be-painted object, to thereby form an electric field and charge the paint particles passing through the discharging area and carry them on an electric field formed toward the object, thereby promoting the paint particles to well adhere to the object. In comparison with the aforementioned direct charging technique, however, the paint particles passing through the discharging area cannot be charged so effectively as to assure a sufficient electrostatic effect.
Generally, the direct charging used mainly with a solvent-type paint is adopted in an electrostatic-painting spray gun usable effectively on the commercial base. In this case, the voltage applied to the electrode is on the order of −30 to −70 kV. The larger the potential difference, the greater the electrostatic effect is. However, because of a greater danger due to a spark discharge or shock discharge with application of a high voltage and the dielectric-strength design of the apparatus or the like, the applied voltage is required to be as low as possible. Therefore, the voltage applied to the electrode is about −50 kV in many cases. On the other hand, normally in the external charging, the electrode is applied with a voltage having a larger potential difference. In comparison with the direct charging technique in which the charging electrode is disposed in the center of paint spray for efficient charging, the external charging technique needs a higher voltage for a greater practical effect and the charging electrode is disposed ahead of the atomizer to prevent any dangerous discharging to the paint spray nozzle as a substantial ground potential side of the apparatus, and drop of the voltage caused by the discharging at the charging electrode.
Generally, the commercial-use spray gun is designed so that the charging electrode is positioned 80 to 150 mm ahead of the paint spray nozzle. Namely, the charging electrode is projected largely ahead of the paint spray nozzle. Therefore, many of the commercial-use spray guns are used as a hand-held automatic spray gun connected to, and driven by, an automatic painting apparatus in many cases. More specifically, in this hand-held spray gun, the charging electrode is projected largely ahead and positioned aside off the center axis of spraying to prevent the function from being lessened due to adhesion of the sprayed paint, with the result that the spray gun is a large and not easy to handle and operate. Thus, the worker engaged in painting with this spray gun will be correspondingly more burdened.
Also, a hand-held type electrostatic-painting spray gun is disclosed in the Japanese Patent Application Laid Open No. 30646 of 1978. In this spray gun, the electrode projected to the forward end of the gun leads to a poor operability and is likely to be broken due to collision during operation. These problems caused this spray gun not to be more popular. Also, in comparison with the direct charging technique in which the paint particles are directly charged, the extent of charging by discharging from the electrode disposed outside is rather smaller. Thus, the external charging technique is required to use a higher voltage for a higher degree of charging while assuring a higher safety and improve the efficiency of paint adhesion by the effective charging.
Since the aforementioned external charging cannot assure any sufficient charging, consideration should be given to the use of a higher voltage, measures against a danger and dielectric breakdown possibly caused by the application of the high voltage. Namely, the apparatus of this external charging type has many problems to solve, such as a compact design and practical applicability thereof as a hand-held spray gun.
Also, the painting spray guns including the electrostatic-painting spray gun and other types of spray guns are used each as an automatic spray gun in painting a series of many objects in a lot in an automated line of production in a factory. In a post along the production line where there is effected a painting upon which the quality of the paint film depends, the worker uses a hand-held spray gun in many cases. However, many of the spray guns of this hand-held type are problematic in safety and operability. Namely, improved ease of handling and operation of the spray gun leads directly to an improved economical efficiency such as higher efficiency of paint adhesion as well as to improved work efficiency, quality and stability of painting, which will be extremely important factors from the overall and long-term viewpoints.
The conventional electrostatic-painting spray guns are designed each for use as an automatic spray gun mounted on an automatic painting apparatus. Therefore, the hand-held type electrostatic-painting spray gun should be designed easier to handle and operate, strictly safer against possibly direct danger to the worker, and more compact.
Any external electrode to be provided in a conventional hand-held spray gun should be disposed largely ahead of the paint spray nozzle. Therefore, it is essential that the electrode should be lightweight. However, major consideration being given to a combination of necessary members in the spray gun, the problems for easier handling and operability of the spray gun, which are most important for the worker using the spray gun, have not yet been solved.
Also, in the case of the conventional hand-held type electrostatic-paining spray gun, a high-voltage cable from an external high-voltage generator is connected to the spray gun for receiving a high voltage. However, there have not yet been solved the problems such as incorporation, integrally in the spray gun, of the high-voltage generator to apply a voltage to the electrode in order to free the worker from the weight of the high-voltage cable and prevention of a danger of the high voltage applied to the electrode disposed ahead of the paint spray nozzle. The spray gun should be a comprehensively high-practicality one in which such solutions are implemented.
Also, if a paint is continuously sprayed with the paint being adhering to the external electrode and electrode holder or receptacle, the sprayed paint particles will be heaped up and drip off, which will spoil the quality of a finish coat. Of course, the spray gun with such a result is not reliably usable. In case a paint easily solidified and not soluble with the solvent, such as a water-based paint, is used, the solid content of the paint will act as an insulator and adhesion thereof to the electrode will greatly degrade the action and effect of the electrode. Therefore, the electrostatic-painting spray gun should be durable for a long term of use and able to readily be restored to its normal condition even if it should have the function thereof lessened or disabled.